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Air vs. Steam Atomization Part 8 – Economics (Plant Air Production)

Posted on October 5th, 2017 by in Chemical Manufacturing Excellence

Air-Steam-Atomization-8

Previous posts discussed incineration, atomization and associated stack losses. The air compressor is loaded for approximately 30 seconds while drawing 400 kW of electrical energy and unloaded for approximately 60 seconds while drawing 60 kW. One-third of the compressor capacity is being utilized and the electrical energy required to produce compressed air for the plant will be approximately 173 kW (30*400+60*60)/(30+60).

Note that the compressor is wasting energy when unloaded because it is consuming 60 kW while not producing any compressed air. Operating the compressor continuously loaded at a slower speed will reduce this inefficiency significantly because the compressor would (theoretically) operate at one-third speed and draw 133 kW.

The economic difference between the existing load/unload operation and the proposed variable speed drive operation is approximately 40 kW (173 – 143 kW), which is approximately USD 20,000 annually when the marginal annual cost of electricity is approximately USD 500 per kW.

Note that this economy is possible when atomizing with air when the compressor continuously operates loaded at over 50 percent speed (so as to not damage the compressor).

Another subtlety with regard to operating constant torque loads (such as positive displacement air compressors) is that the motor remains fully loaded at reduced speeds. Fully loading the motor with non-sinusoidal waveforms (such as generated by a variable speed drive) tends to create more heating in the motor when it is running at reduced speed – potentially with reduced cooling. More rugged motors are often used in these applications such as TENV (totally enclosed non-ventilated) motors to compensate for reduced cooling that would occur in TEFC (totally enclosed fan cooled) motors. Often higher temperature windings are used to withstand the effects of the additional heat.

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All opinions shared in this post are the author’s own.

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